There is conventionally known a fixing device for thermally fixing a developer image to a recording sheet such as paper. One such fixing device is provided with an endless fusing belt, a nip member disposed inside a loop formed by the fusing belt, and a backup member such as a pressure roller that pinches the fusing belt in cooperation with the nip member. In order to improve sliding characteristics between the nip member and the circulating fusing belt in the fixing device having this construction, lubricant is provided between the fusing belt and the nip member.